Kubica, who required five operations following a serious rallying accident in February, has been making steady progress from his injuries but crucially still lacks strength in his right arm, which was badly damaged in the crash.
The Pole's manager, Daniele Morelli, recently confirmed Kubica would step up his rehabilitation by getting back behind the wheel of either a road car or simulator, but Boullier has revealed he has not yet been given any indication of when a return to the cockpit is likely.
An original mid-October deadline for an answer on Kubica's potential involvement in 2012 has already been extended, and time is now pressing on Boullier.
"We have to assess options and we are still waiting for Robert to come back to us to say he can test," Boullier said in quotes reported by www.autosport.com.
"The last information I have is that we will have to sit down and discuss how we can handle his comeback, because I don't believe he can commit in the coming days. In the coming weeks maybe - but not coming days.
"So we have to make sure that we work together on a plan for him to come back."
"We have everything ready. The 2009 cars are ready in the garage, ready to fire up. We are ready for him but we are waiting for him to tell us he is ready.
"I know they are very confident and I am happy about this, because I am the first one wishing to have Robert back in the car, but nobody knows.
"Nobody has a medical report and nobody has seen him driving a car and being at speed. Until we see him and have the confirmation then, we can not say anything. And we need to keep, unfortunately, the option of not having him back."
Renault have a number of options for their 2012 driver line-up should Kubica fail to return.
Current drivers Vitaly Petrov and Bruno Senna are both in the frame, while Romain Grosjean, who will drive for Renault in the first practice sessions at the final two races of the season in Abu Dhabi and Brazil, is also pushing for a race seat.